If we are successful, in one great step our two nations will have been able to ease one of the greatest sources of tension in international affairs: that between the developed and developing worlds.

【翻译】如果我们取得成功的话，在很大程度上，我们两个国家将会缓解国际事务中最紧张局面的一个根源，这便是：发达国家和发展中国家之间的紧张局面的根源。

【分析】If we are successful（条件从句）, in one great step our two nations will have been able to ease one of the greatest sources of tension in international affairs: that between the developed and developing worlds.（主句）

重点分析主句：

in one great step （程度状语）our two nations （主语）will have been able to ease （谓语）one of the greatest sources of tension in international affairs（宾语）: that between the developed and developing worlds.（同位语。that 指代one of the greatest sources of tension in international affairs。同时，between the developed and developing worlds 作定语修饰that ）

will have been able to...：将来完成时，表示“将来业已完成的动作”，表示说话者对所给条件付诸实施后的一种判断。